This invention relates to corrosion inhibiting compositions and more particularly to corrosion inhibitors for use in brines.
Brines, for example, those referred to as "clear brine fluids," such as aqueous solutions of alkali or alkaline-earth metal halides including CaCl.sub.2, CaBr.sub.2, MgCl.sub.2 and mixtures thereof, are used as drilling, completion, packer and workover fluids in oil and gas well operations. These brines are useful in providing a hydrostatic head, particularly in high-pressure wells which typically require fluids with densities of 8.5 pounds per gallon (ppg) or above. The choice of the specific salt employed in preparing the clear brine fluid depends on the required density for the particular end-use application. For example, brines having densities up to about 15 ppg are generally prepared from CaCl.sub.2 or CaBr.sub.2, whereas brines having a higher density, of above about 15 ppg, may be prepared using a mixture of, for example, a calcium halide and a significant percent of a zinc-based salt, generally more than about 5 percent by weight. Although lower density brines could be prepared using zinc salts, due to the relatively high costs of zinc salts these brines are most commonly prepared a calcium salt only. The use of these brines has improved well drilling, completion, packer and workover operations by eliminating formation plugging problems and solid settling problems previously encountered when solids-containing media, such as water base and oil base drilling muds, were used.
In spite of the improvements provided by the use of clear brine fluids, clear brine fluids can be corrosive under operating conditions in drilling, completion, packer and workover operations. The corrosion problem is exacerbated by the higher temperatures typically found in deeper wells in which these brines are used.
The prior art teaches that various corrosion inhibitors can be added to brines in an effort to overcome the corrosive nature of brines. Known inhibitors include, for example, film-forming amine-based corrosion inhibitors such as are described in U.S. Pat. No. 4,292,183.
Thiocyanate or thiourea compounds, used alone or in combination with quaternary pyridinium, quinolium or isoquinolinium salts, are known to be effective corrosion inhibitors for a number of aqueous brines of low and high densities, including those comprising calcium chloride, calcium bromide, calcium iodide, as well as various high density zinc-based brines. This is disclosed in British Pat. No. 2,027,686A.
However, while various effective corrosion inhibitors are known in the art, the present invention provides a brine composition showing improvements over the known art in achieving corrosion inhibiting properties that are both measurable and predeterminable, as well as a method of inducing corrosion inhibition of metals.